The present Applicant has developed a plethora of thermal bubble-forming printheads and thermal bend-actuated printheads. The Applicant's thermal bubble-forming printheads include those with suspended heater elements (as described in, for example, U.S. Pat. Nos. 6,755,509; 7,246,886, 7,401,910; and 7,658,977, the contents of which are incorporated herein by reference) and those with embedded heater elements (as described in, for example, U.S. Pat. Nos. 7,377,623; 7,431,431; US 2006/250453; and U.S. Pat. No. 7,491,911, the contents of which are incorporated herein by reference). The Applicant's thermal bend-actuated printheads typically have moveable paddles defined in a nozzle plate of the printhead (as described in, for example, U.S. Pat. Nos. 7,926,915; 7,669,967; and US 2011/0050806, the contents of which are incorporated herein by reference).
One characteristic of many of the Applicant's inkjet printheads is a nozzle chamber and/or a nozzle plate comprised of silicon nitride. The nozzle plate spans across an ink ejection face of the printhead and defines a roof of each nozzle chamber in the printhead. It will be appreciated that the roof of each nozzle chamber, as well as chamber sidewalls, have surfaces that are continuously in contact with ink contained in each nozzle chamber. In some printheads, the nozzle chamber roof may comprise a bi-layer of silicon nitride and silicon oxide (see, for example, U.S. Pat. No. 7,658,977 and US 2011/0050806). In these printheads, the lower silicon nitride layer has a surface which is exposed to ink in the nozzle chamber. In other printheads (e.g. U.S. Pat. No. 6,755,509), the nozzle chamber roof may consist of a mono-layer of silicon nitride.
Silicon nitride is an excellent material for use in fabricating nozzle chambers and nozzle plates in inkjet printheads. Silicon nitride has excellent mechanical robustness under high pressures, good resistance to cracking and can be deposited by PECVD, which is compatible with conventional MEMS fabrication techniques.
Notwithstanding the excellent mechanical properties of silicon nitride nozzle plates, the present Applicant has observed an apparent corrosion of silicon nitride when exposed to certain dye-based inks over prolonged periods (e.g. 6-12 months). In the Applicant's bi-layered nozzle plate structures, comprising a lower layer of silicon nitride and an upper layer of silicon oxide, an unacceptable degree of roof delamination has been observed. This delamination results in a significant reduction in print quality, especially in black ink channels where delamination is most severe. Further SEM investigation of delaminated printheads revealed that the silicon nitride layer of each roof had apparently corroded whereas the silicon oxide layer was left relatively intact. However, some corrosion of silicon oxide was also observed, albeit at a relatively slower rate of corrosion than silicon nitride.
Another characteristic of the Applicant's inkjet printheads is the integration MEMS and CMOS layers in a single printhead integrated circuit (IC), which has enabled the development of inkjet printheads having a high nozzle density using standard semiconductor fabrication techniques. In the Applicant's printhead ICs, it is necessary for ink to flow from a backside of the printhead IC, which receives ink from a molded ink manifold, to a frontside of the printhead IC containing the MEMS nozzle chambers. Therefore, the ink must pass through ink inlets defined in the CMOS layers. Clearly, if ink comes into contact with any CMOS Metal layers then this is potentially catastrophic for printhead operation.
In a typical CMOS design, a lowermost Metal 1 CMOS layer is disposed on a BPSG layer. This BPSG layer has an edge exposed to ink via the ink inlet. Although many inks do not affect this BPSG layer (or other silicon oxide layers) in the printhead IC, it has been found that some inks cause significant corrosion of the BPSG layer which is problematic for printhead longevity. Relatively slower corrosion of a CVD oxide interlayer dielectric was also observed in some instances.
From the foregoing, it will be apparent that there are a number of structures in printhead, which are potentially corrodible by exposure to dye-based inks at typical pHs (e.g. pH 6-8).
One possible solution to the corrosion problems discussed above, which is currently under investigation by the present Applicant, is to physically isolate the silicon nitride or BPSG layer from the ink. For example, the nozzle plate may be designed so that a protective barrier layer is disposed between the silicon nitride and the ink. Alternatively a protective collar may be formed around the inner surfaces of each ink inlet. However, this type of mechanical solution to the problem of corrosion has the significant drawback that it requires a more complex printhead design, as well as the development and optimization of suitable MEMS fabrication processes. On a practical level, redesigning printheads is highly undesirable, especially when optimized printhead fabrication processes are well-established and suitable for mass-production.
It would therefore be desirable to seek an alternative solution to the problem of corrosion in printheads by dye-based inks, which does not require modifying the design of the printhead.
US 2010/0271448 describes dissolution of silicates from elemental silicon in printheads and identifies high pH pigment-based inks as the cause of this dissolution. US 2010/0271448 proposes the use of trivalent aluminium for passivating native silicon surfaces and suppressing the dissolution of silicates caused by high pH pigment-based inks Consistent with the present Applicant's observations, US 2010/0271448 reports that no dissolution of silicates is observed when silicon printhead dies are exposed to dye-based inks, and the addition of trivalent aluminium, therefore, has no effect in such systems.